Amyotrophic lateral sclerosis (ALS; also known as Lou Gehrig's disease and motor neuron disease) is a progressive, fatal, adult-onset degenerative disorder of motor neurons in the primary motor cortex, corticospinal tracts, brainstem and spinal cord, leading to paralysis of voluntary muscles. Currently, the incidence and prevalence of ALS are 1-2 per 100,000 each year, respectively, with a lifetime ALS risk of 1 in 400 to 1 in 1,000. Most incidences of ALS are sporadic but approximately 10% of patients have a familial history (known as familial ALS; fALS). In both sALS and fALS, there are progressive manifestations of dysfunction of the lower motor neurons and cortical motor neurons but without sensory symptoms. Age and gender are documented sALS risk factors with a male-to-female ratio of 3:2 among the patients. Among the mutations associated with ALS, those in the Copper-Zinc superoxide dismutase (SOD1) gene have long been thought to cause the ALS disease through a toxic gain of function rather than impairment of the antioxidant function of the SOD1 enzyme. Other genes with mutations associated with the fALS include alsin (ALS2), senataxin (ALS4), vesicle associated membrane protein (VAPB, ALS8), Angiogenin and the p150 subunit of dynactin (DCTN1). Recently, more than thirty mutations in the TDP-43-coding region of Tardbp have been identified in ALS patients with or without apparent family history, corresponding to approximately 4% of fALS and less than 1% of sALS. Most patients with TDP-43 mutation(s) develop a classical ALS phenotype without cognitive deficit suggesting an important role of TDP-43 in the development of ALS.
TDP-43, or TAR DNA-binding protein-43, is a ubiquitously expressed nuclear protein encoded by one of the mRNA isoforms from the highly conserved Tardbp gene. It is a RNA-binding protein involved in transcriptional repression, pre-mRNA splicing, and translation. TDP-43 has also been identified as the major pathological signature protein of intracellular inclusions typical for disease cells of a range of neurodegenerative diseases, including frontotemporal lobar degeneration with ubiquitin-positive, tau- and α-synuclein-negative inclusions (FTLD-U) and amyotrophic lateral sclerosis (ALS). The TDP-43 molecules in the diseased cells of the patients' brains or spinal cords are characterized by abnormal ubiquitination, hyperphosphorylation and partial cleavage to generate ˜−25 kDa and 35 kDa C-terminal fragment(s). Furthermore, TDP-43 is partially or completely cleared from the nuclei of either neuronal or glial cells containing the TDP-43 (+) and ubiquitin (+) aggregates/inclusions, or UBIs, in the cytoplasm.
Several mouse models have been established for ALS disease, which include strains of rodents that are transgenic with SOD1, ALS2-knockout mice, and mice with genetically engineered genes coding for the neurofilament subunits. Among these, the human mutant SOD1 (mSOD1) transgenic mouse model is currently the most widely used one because it shares several clinical phenotypes with ALS patients. The first symptom of mSOD1 mice is a fine “jittering/tremor” in one or more of the limbs, which appears at approximately 90 to 100 days of age. At later stages, the mice begin a clinical course, first with muscle weakness and/or paresis in the hind limbs, followed by ascent of paresis to the forelimbs and finally severe quadriplegia. The cytopathological features of the mSOD1 Tg mice include motor neuron loss with astrocytosis, the presence of SOD1-positive inclusions including Lewy body-like hyaline inclusions/astrocyte hyline inclusions, and vacuole formation. Among these three pathologic features, motor neuron loss with gliosis is the most essential one shared between the mSOD1 Tg mice and ALS.
Overexpression of TDP-43 in transgenic rodents could also lead to development of motor neuron disease-like symptoms. These Tg rodents develop one or more of several symptoms, which include motor neuron dysfunction, muscle defect-related pathology such as spastic paralysis, and neuronal loss. The life spans of some of these Tg mouse lines are short, likely due to the relatively low motor-neuron specificity of the promoters used, e.g., Thy1, prion, etc. Finally, the appearance of cells with cytoplasmic TDP-43(+) UBIs and TDP-43 depleted nuclei at later stages of pathogenesis of the TDP-43 Tg mice suggest that the disease phenotypes in the TDP-43 Tg mice may result in part from loss-of-function of TDP-43. However, the pathological phenotypes of the mice could also be caused entirely by gain-of-toxicity from overexpression of the exogenous TDP-43. Thus, the relative contributions of loss-of-function and gain-of-cytotoxicity to the neurodegeneration in FTLD-U and ALS with TDP-43 (+) UBIs are not clear. In addition, regardless of its currently known biochemical and structural properties, the physiological functions of TDP-43 in different mammalian tissues are also unknown.
A previously unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies, especially in connection with the roles of TDP-43 in ALS.